Computerized axial tomography systems or machines, i.e., CAT scanners, have been used for a number of years to obtain three-dimensional images from two-dimensional data. These systems have been used primarily by the medical profession to obtain three-dimensional pictures of the body, but they also have been used in industry for a variety of purposes.
Typically, the system comprises a source of x-rays, a sample positioned in the path of the x-rays, a detection system and an analyzing computer.
Conventional computerized tomography (CT) measurements for industrial purposes involve collecting absorption information for a single cross-sectional slice through a material. Spatial resolution is achieved by either collimating an incident beam using a pinhole and then rastering the beam across the sample, or by using a position sensitive-linear photodiode array to measure all the projection data for a single angular view in parallel.
There are disadvantages to both of these systems. The primary disadvantage of using the pinhole is that most of the incident radiation is thrown away. As a consequence, the rastering technique is extremely time consuming. Acquiring the data for reconstruction of a single sample takes upwards to twelve hours.
The photodiode array is noisy and subject to non-linearities. The noise limits the dynamic range, and therefore, the maximum contrast that can be studied in a sample. The non-linearities also introduce ring-like artifacts in the reconstructions which can further reduce the usefulness of the information obtained. Finally, it still requires days to obtain enough information for three-dimensional sample visualization.